Fuel Problems, HELP!

[jgasca]

Wait, wait - I was just showing you the GM recommended changes - I don't believe that making them will solve your problem.

Remember, your truck was running fine until just recently - with the same timing/settings you've always had.

I'm telling you - look elsewhere for the cause. I seriously doubt that altitude or carburetion is your problem. A chevy 350 engine that is in decent shape internally can take a lot of abuse and will still run pretty good.

Think about it, you are in your home town and the truck is humming along for a few days and then all of a sudden it starts running rough - it's not a timing or altitude issue.

Do you have a friend that would let you take his carb off and put it in your truck for a test?

I still think it's either an ignition or fuel flow problem.

Well I had time yesterday afternoon, so I took the carb apart and drenched the whole thing in carb cleaner. I noticed one of the float "needle" was sticking a bit......could this be my problem? I cleaned it the best I could so lets see. I put it back together and fired her up. Ran smooth, took it out for a drive and it ran pretty good.....I advanced my timing to 12 degrees and set the idle down by ear.

Jerry I understand what your saying, what I find completely odd is it runs fine every time I clean out the carb, Im not saying I found any dirt or clogs in the carb this time around, Could this be a bad tank of gas that just pushed a dirty carburator to its limits.....

I'll be driving it this weekend all I can to give it a good test. I'll keep you guys posted.

 

[Boone83K10]

yeah why dont you just drain and clean the tank... use that gas in the lawnmower. Eliminate that possibilty.

 

[jgasca]

yeah why dont you just drain and clean the tank... use that gas in the lawnmower. Eliminate that possibilty.

Yeah I was thinking that. I was even thinking of dropping the tank and seeing what it looks like inside. This truck was parked for a long while before I got it so the tank may be a disaster. Any advice on inspecting the tank?

 

[Boone83K10]

Use siphon to get gas or through filter neck. Unbolt the brace brackets from frame, 4 bolts each brace bracket. Do not unbolt the strap retaining nut. Lower tank, unhook fuel lines and ground wire. Use hammer and screwdriver to take off lock ring. Pull sending unit out.

 

[chengny]

Jose,

This is a quality fuel filter you have in line between the tank and the carburetor:

You must be registered for see images attach

Think about it - the only way anything would get out of the gas tank and into the carb is if the element was torn or burst open. It would be really, really rare if you got a bad filter out of the box. Also a good determination of the gas tank's condition can be made by looking through the glass and noting whether or not the filter is picking up any rust or varnish.

If it is packed with crap, your tank is dirty and the debris in the filter would eventually restrict fuel flow. But, unless the filter was broken, it is still doing it's job and will not allow anything to reach the carb.


If it's clear your tank is clean and the filter is allowing fuel to pass. You can eliminate the tank and a clogged filter.

While we are on the subject of fuel flow; have you done a test of the fuel pump's output (flow rate and pressure)?

FUEL PUMP FLOW TEST


1.Connect hose from carburetor fuel feed line to a suitable unbreakable container and crank engine.
2.Fuel pump should supply 1/2 pint or more in 15 seconds.
3.If flow is below minimum, check for fuel restriction. If there is no restriction, check pump pressure and/or vacuum.

FUEL PUMP PRESSURE TEST


1.Disconnect the fuel line at the carburetor.
2.Install a low pressure gage onto the line.
3.Start the engine.
4.Fuel pump pressure should be (4 to 6.5 psi) .
5.If pressure is less, check for restrictions in fuel tank sender filter, lines or hoses.
6.Reconnect the fuel line

To run the engine during the pressure test there are 2 ways:

You can tee a gage into the gas line (involves cutting into the gas line between the pump and carb - pain in the ssa)

Or disco the line from the carb and dead-end a gage on it. If you do it this way (it is a lot easier and faster) you need to keep the engine running with only the fuel remaining in the carb. Get your pressure reading quick before the gas runs out. Alternately you can keep it running on ether.

 

[Jims86]

A good idle speed to shoot for is 550 in gear.....also, set your timing in gear while someone steps on the brake, will keep the advance weights retracted and so you can set an accurate base.

 

[jgasca]

Jose,

This is a quality fuel filter you have in line between the tank and the carburetor:

You must be registered for see images attach

Think about it - the only way anything would get out of the gas tank and into the carb is if the element was torn or burst open. It would be really, really rare if you got a bad filter out of the box. Also a good determination of the gas tank's condition can be made by looking through the glass and noting whether or not the filter is picking up any rust or varnish.

If it is packed with crap, your tank is dirty and the debris in the filter would eventually restrict fuel flow. But, unless the filter was broken, it is still doing it's job and will not allow anything to reach the carb.


If it's clear your tank is clean and the filter is allowing fuel to pass. You can eliminate the tank and a clogged filter.

While we are on the subject of fuel flow; have you done a test of the fuel pump's output (flow rate and pressure)?

FUEL PUMP FLOW TEST


1.Connect hose from carburetor fuel feed line to a suitable unbreakable container and crank engine.
2.Fuel pump should supply 1/2 pint or more in 15 seconds.
3.If flow is below minimum, check for fuel restriction. If there is no restriction, check pump pressure and/or vacuum.

FUEL PUMP PRESSURE TEST


1.Disconnect the fuel line at the carburetor.
2.Install a low pressure gage onto the line.
3.Start the engine.
4.Fuel pump pressure should be (4 to 6.5 psi) .
5.If pressure is less, check for restrictions in fuel tank sender filter, lines or hoses.
6.Reconnect the fuel line

To run the engine during the pressure test there are 2 ways:

You can tee a gage into the gas line (involves cutting into the gas line between the pump and carb - pain in the ssa)

Or disco the line from the carb and dead-end a gage on it. If you do it this way (it is a lot easier and faster) you need to keep the engine running with only the fuel remaining in the carb. Get your pressure reading quick before the gas runs out. Alternately you can keep it running on ether.

Thanks for confirming, I wasn't sure if that filter was good quality or not. So I already see some debree in the filter. Thats saying its only been about 2 weeks that I've installed it and I haven't even put more than 200 miles on the truck since. I'll keep an eye on it though. Ill be adding an inline gauge to check my pressure.

What happens in the event that the fuel pressure is above 6.5?


Also is this tach good enough for my needs?

http://www.autozone.com/autozone/ac...achometer/_/N-25x9?itemIdentifier=477562_0_0_

Or could you guys recommend me one.... thanks!

 

[chengny]

What happens in the event that the fuel pressure is above 6.5?

It won't be. Mechanical fuel pumps are self regulating. When the discharge pressure rises above a certain point, the diaphragm is prevented from deflecting fully. The higher the discharge pressure, the less fuel is displaced per stroke.

If the carb floats shut off fuel flow completely, the pressure on both sides of the diaphragm eventually equalizes and the pump just idles. When the carb calls for more fuel, the pressure in the discharge line falls off, the diaphragm can start to flex and fuel starts to move again.


Also is this tach good enough for my needs?

You must be registered for see images attach

That is great. Sunpro makes good instrumentation - that tach is all you need to check engine speed while making repairs/adjustments. Tachometers mounted on the dashboard of pick-up trucks are for posers. When do you want to check RPM? While tuning the engine. Why have an RPM indicator mounted where you can't see it?

A couple of places I have mounted tachs.


On the blower motor housing:

You must be registered for see images attach

On the hydroboost accumulator cylinder:

You must be registered for see images attach

 

[jgasca]

What happens in the event that the fuel pressure is above 6.5?

It won't be. Mechanical fuel pumps are self regulating. When the discharge pressure rises above a certain point, the diaphragm is prevented from deflecting fully. The higher the discharge pressure, the less fuel is displaced per stroke.

If the carb floats shut off fuel flow completely, the pressure on both sides of the diaphragm eventually equalizes and the pump just idles. When the carb calls for more fuel, the pressure in the discharge line falls off, the diaphragm can start to flex and fuel starts to move again.


Also is this tach good enough for my needs?

You must be registered for see images attach

That is great. Sunpro makes good instrumentation - that tach is all you need to check engine speed while making repairs/adjustments. Tachometers mounted on the dashboard of pick-up trucks are for posers. When do you want to check RPM? While tuning the engine. Why have an RPM indicator mounted where you can't see it?

A couple of places I have mounted tachs.


On the blower motor housing:

You must be registered for see images attach

On the hydroboost accumulator cylinder:

You must be registered for see images attach

Alright cool. One other thing, I noticed last night that I could smell gas from the exhaust...lots of it. I'm thinking my fuel to air ratio is way off. Any advice on adjusting that? Am I correct in saying that the only way to run leaner in an edelbrock is changing jets to match the calibration desired?

 

[chengny]

Well, yeah. From what I can read you were kind of blindly adjusting the mix screws way in and out... you are probably far off spec.

Start from scratch. Here is the idle mixture adjust procedure for the Edelbrock Performer Series:

You must be registered for see images attach

The complete manual for those carburetors (DF, no PW):


http://depositfiles.com/files/59jdrujlm

 

[jgasca]

Ok so first thing I did today was hooked up the fuel pressure gauge between the pump and the carb.....Fired up the truck and gauge started pulsating. Did some research and read that this happens with mechanical pumps.......

This is what it looks like

You must be registered for see images attach

So looks to me like it bounces from 0 to 8 psi........is it safe to say that I'm at around 4 psi at idle???

If so is this too low?

 

[chengny]

Jeez Jose - IDK

There will be some fluctuation in the discharge pressure of a mechanical FP... but to see the needle bouncing from 0 -8 psi. I think that may indicate a problem.

To be totally honest I never have to go beyond the flow test. My way:

The distributor is unplugged/disabled to prevent the engine from starting

I disco the gas line from the carb, put a hose on the end of the steel tubing and stick the end of the hose into a mason jar. While my assistant (wife, one of my kids, whoever happens to walk by) cranks the engine, I hold the hose in the jar and watch the flow.

I check for 3 things:

1. A steady flow (a little pulsing is expected)

2. Sufficient quantity of fuel delivered (almost fill the jar in at most 15 seconds)

3. That no air is present in the fuel that comes out (keep the end of the hose below the surface of the collected gasoline and watch for bubbles)

If you are checking pressure with an in-line gauge and the engine is idling, I would expect a more stable reading than bouncing from 0-8 psi.

Do you have a return/vent line connected to the gas pump? Try pinching it off (gently) wit a pair of vise-grips and see if you get a steadier reading.

Mechanical fuel pumps hardly ever fail. But two things can happen:

1. The diaphragm can split (usually results in 0 pressure)

2. One of the check valves can stick open (causes back-flow through the pump)


If a check valve is hung open, the pressure that the diaphragm creates (in the discharge line) on it's upward stroke, is lost when the diaphragm is deflected back down by the spring.

So instead of building up and maintaining a consistent discharge pressure, the pressure shoots up and then is dumped back to the tank via the suction line on every stroke.

This can cause the float valves in the carburetor to cycle open and closed as the floats move up and down.

Another cause of pressure fluctuation is a problem in the fuel pump suction line - between the tank and the pump inlet. Again, two main issues to look for:

1. A weak or partially collapsed section of rubber hose

2. A split or loose connection in the suction line that allows air to be drawn in and mix with the gas.

It is often believed that if there is no gas on the driveway there can be no leak in the suction line. Not true; there are sections of the gas line that are above the tank top and well above the fuel pump. If there is a leak in the piping in that area the fuel pump will suck air in while operating. But when it stops, the gas in the line at the point of the leak will either kind of just hang there (if the leak is in a rubber hose the rubber will close it back up) or drain back into the tank.


BTW - how has the engine been running lately?

 

[jgasca]

Jeez Jose - IDK

There will be some fluctuation in the discharge pressure of a mechanical FP... but to see the needle bouncing from 0 -8 psi. I think that may indicate a problem.

To be totally honest I never have to go beyond the flow test. My way:

The distributor is unplugged/disabled to prevent the engine from starting

I disco the gas line from the carb, put a hose on the end of the steel tubing and stick the end of the hose into a mason jar. While my assistant (wife, one of my kids, whoever happens to walk by) cranks the engine, I hold the hose in the jar and watch the flow.

I check for 3 things:

1. A steady flow (a little pulsing is expected)

2. Sufficient quantity of fuel delivered (almost fill the jar in at most 15 seconds)

3. That no air is present in the fuel that comes out (keep the end of the hose below the surface of the collected gasoline and watch for bubbles)

If you are checking pressure with an in-line gauge and the engine is idling, I would expect a more stable reading than bouncing from 0-8 psi.

Do you have a return/vent line connected to the gas pump? Try pinching it off (gently) wit a pair of vise-grips and see if you get a steadier reading.

Mechanical fuel pumps hardly ever fail. But two things can happen:

1. The diaphragm can split (usually results in 0 pressure)

2. One of the check valves can stick open (causes back-flow through the pump)


If a check valve is hung open, the pressure that the diaphragm creates (in the discharge line) on it's upward stroke, is lost when the diaphragm is deflected back down by the spring.

So instead of building up and maintaining a consistent discharge pressure, the pressure shoots up and then is dumped back to the tank via the suction line on every stroke.

This can cause the float valves in the carburetor to cycle open and closed as the floats move up and down.

Another cause of pressure fluctuation is a problem in the fuel pump suction line - between the tank and the pump inlet. Again, two main issues to look for:

1. A weak or partially collapsed section of rubber hose

2. A split or loose connection in the suction line that allows air to be drawn in and mix with the gas.

It is often believed that if there is no gas on the driveway there can be no leak in the suction line. Not true; there are sections of the gas line that are above the tank top and well above the fuel pump. If there is a leak in the piping in that area the fuel pump will suck air in while operating. But when it stops, the gas in the line at the point of the leak will either kind of just hang there (if the leak is in a rubber hose the rubber will close it back up) or drain back into the tank.


BTW - how has the engine been running lately?

wow I have no idea either. The pump does have a return line connected. I'll try pinching that and see what happens. So lately its been running better. Seems like it bogs a little bit when changing gears at a low idle. Also the exhaust continues to smell alot like gas.

Also I followed your instructions on tuning the mixture screws. I got the highest idle with the driver side screw, then when I tried the same with the other.. it never raised the idle....

Im completely stumped. May just end up taking it to someone, might just have more luck than what I'm having.

 

[chengny]

Once you have the highest idle you can get on one side, manually lower the idle speed with the external idle speed set screw.

When you have it low, but still running, then do the other side. You can't set the idle mixture with the idle up high - it has to be almost dying when you start.